The invention disclosed and claimed herein pertains to structural apparatus for supporting the coils of a gradient coil pair or set for the main magnet of a magnetic resonance (MR) imaging system, wherein the coils are selectively spaced apart from one another and are energized interactively to generate a gradient magnetic field within a specified volume, such as the magnet bore. More particularly, the invention pertains to support structure for gradient coils of such type wherein the current therethrough is substantially increased.
It has become well known in MR imaging to employ a gradient coil pair or set, also known as a self-shielded or Roemer gradient coil set, to generate each of the gradient magnetic fields, i.e., the X-, Y-, and Z-gradient fields. Generally, a gradient coil set has two gradient coils, each wrapped around one of two cylindrical coil forms, one of the forms having a greater diameter than the other, and the forms being in spaced-apart coaxial relationship within the bore of the associated MR main magnet. Each coil of a set is actuated or energized by a current to produce a magnetic field, the two coils being designed in relation to each other so that their fields combine within the magnet bore to produce a resultant magnetic field comprising one of the gradient fields. The fields of the two coils cancel each other out outside the outer coil form, so as not to modify or alter the main magnetic field. A gradient coil pair or set is described, for example, in commonly assigned U.S. Pat. No. 4,737,716, issued Apr. 12, 1988 to Roemer et. al., and for which re-examination certificate B1, 4,737,716 was issued.
It is now common practice in MR imaging to place the inner coils for X-, Y-and Z-gradient coil sets on a single inner coil form, and to place the corresponding outer X-, Y- and Z- coils on a single outer coil form which is in coaxial relationship with the inner form. Thus, the inner and outer coils of each coil set are in spaced apart relationship with each other. In a typical arrangement, each coil of the Z-gradient coil set comprises a wire helically wound around one of the coil forms. Each coil of the X-gradient coil set comprises a fingerprint coil or the like, which is etched or otherwise formed on a sheet or board of copper, each such sheet being wrapped around a coil form over the Z-gradient windings. The Y-gradient coils likewise comprise fingerprint coils formed on copper sheets, one such sheet being wrapped around each coil form over the sheet containing the X-gradient windings, but in orthogonal relationship therewith.
When current pulses are coupled to the coils of a coil set which is contained within the static field of the MR main magnet, mechanical forces are applied thereto, which tend to displace the coils relative to their respective coil forms. Accurate positioning of the coils relative to their forms is critical for proper generation of gradient fields, so that such forces must be opposed to prevent displacement. In the past, the amounts or levels of current coupled to the coils was sufficiently small that the coils could be firmly held in place on the coil forms by means of tape or the like. Now, however, there is increasing interest in substantially raising the level of the current coupled to the coils to achieve significantly higher performance in MR imaging. The anticipated increase in current level would cause sufficiently strong forces to be applied to the coils that the taping method of the prior art would not be capable of holding the coils in place. Mechanical vibration would be likely to occur, which could damage the coils and also generate a much higher level of noise in the surrounding environment. Moreover, the increased current levels produce significant amounts of heat proximate to the coils. The heat must be conducted away from the coils and the magnet bore region, to prevent damage to the coils and related structure, to avoid unwanted changes in the main magnetic field due to heating of magnet components, and also to prevent unacceptable heating of a patient or other subject in the bore.
One approach would be to fill the space between the coil forms with an adhesive such as epoxy. However, in common gradient coil designs, the volume between the gradient coil forms may be too large for the epoxy to cure uniformly, and possess the mechanical material properties to ensure reliable use in MR imaging. Thus, the epoxy could harden in some regions before hardening in other adjacent regions. This could result in cracking of the epoxy, and could also subject adjacent structure to additional forces and stress. Also, the large volume could significantly increase epoxy curing time.